: : It's a military drill instruction. It means that the front-rank man on the right of the squad is the marker for everybody else; he stands still, while the rest of the squad forms up in relation to him. It's generally normal for a military formation to form on the right; but any well-trained squad can, when so ordered, form on the left or the centre. (VSD)

: In situations where there is no actual line-forming, it's sometimes spoken to a complainer or impatient person --- then the meaning is simply that one's complaint is not unique, and one must wait a while for relief.

It was also a line used in Bobby Darin's version of 'Mack the Knife':

Now Jenny Diver, ho, ho, yeah, Sukey Tawdry
Ooh, Miss Lotte Lenya and old Lucy Brown
Oh, the line forms on the right, babe
Now that Macky's back in town